This work continues a longitudinal study of mental and social development in two cohorts of children observed repeatedly between 2 months and 4 years of age. The premise of this research is that learning in childhood occurs primarily within an interpersonal context; thus, a central goal is to decribed how diverse infant and maternal behaviors relate to the ontogeny of central dimensions of children's mental and social competencies. Children participating in the assessment constitute longitudinal cohorts seen (at least) at 2, 5, 13, 20, and 48 months of age. Major objectives of this research are to study stability, continuity, interaction, and predictive validity in mental and social development over the first years of life.